It's often used to describe someone who is working non-stop, with no time to rest or take a break. The phrase is a metaphor for someone working so hard that they are "flat out" on the floorboards, completely exhausted.
Here are some examples:
* "I'm flat out to the boards with work at the moment, I haven't had a moment to myself."
* "The cafe was flat out to the boards all day, with customers queuing up for coffee."
* "She's been flat out to the boards studying for her exams, but it's all paying off."
The phrase is often used humorously to describe someone who is working very hard, even if they are not literally working on the floorboards. It is a casual and friendly expression that is commonly understood by Australians.